In the construction of many buildings, a finished ceiling, which is referred to as a drop ceiling, is spaced below a structural floor panel that is constructed of concrete, for example. Light fixtures as well as other items appear below the drop ceiling. The space between the ceiling and the structural floor from which it is suspended serves as a return-air plenum for elements of heating and cooling systems as well as a convenient location for the installation of communications cables including those for computers and alarm systems. The latter includes communications, data and signal cables for use in telephone, computer, control, alarm and related systems. It is not uncommon for these plenums to be continuous throughout the length and width of each floor. Also, the space under a raised floor in a computer room is considered a plenum if it is connected to a duct or to a plenum.
When a fire occurs in an area between a floor and a drop ceiling, it may be contained by walls and other building elements which enclose that area. However, if and when the fire reaches the plenum, and if flammable material occupies the plenum, the fire can spread quickly throughout an entire story of the building. The fire could travel along the length of cables which are installed in the plenum if the cables are not rated for plenum use. Also, smoke can be conveyed through the plenum to adjacent areas and to other stories.
A non-plenum rated cable sheath system which encloses a core of insulated copper conductors and which comprises only a conventional plastic jacket may not exhibit acceptable flame spread and smoke evolution properties. As the temperature in such a cable rises, charring of the jacket material begins. Afterwards, conductor insulation inside the jacket begins to decompose and char. If the jacket char retains its integrity, it functions to insulate the core; if not, it ruptures either by expanding insulation char, or by the pressure of gases generated from the insulation exposed to elevated temperature exposing the virgin interior of the jacket and insulation to elevated temperatures. The jacket and the insulation begin to pyrolize and emit more flammable gases. These gases ignite and, because of air drafts within the plenum, burn beyond the area of flame impingement, propagating flame and generating smoke and possibly toxic and corrosive gases.
As a general rule, the National Electrical Code (NEC) requires that power-limited cables in plenums be enclosed in metal conduits. The initial cost of metal conduits for communications cables in plenums is relatively expensive. Also, conduit is relatively inflexible and difficult to maneuver in plenums. Further, care must be taken during installation to guard against possible electrical shock which may be caused by the conduit engaging any exposed electrical service wires or equipment. However, the NEC permits certain exceptions to this requirement provided that such cables are tested and approved by an independent testing agent such as the Underwriters Laboratories (UL) as having suitably low flame spread and smoke-producing characteristics. The flame spread and smoke production of cable are measured using UL 910, Standard Test Method for fire and Smoke characteristics of Electrical and Optical-Fiber Cables Used in Air-Handling Handling Spaces. See S. Kaufman "The 1987 National Electric Code Requirements for Cable " which appeared in the 1986 International Wire and Cable Symposium Proceedings beginning at page 545.
One prior art plenum cable which includes a core of copper conductors is shown in U.S. Pat. No. 4,284,842 which issued on Aug. 18, 1981 in the names of C.J. Arroyo, N.J. Cogelia and R.J. Darsey. The core is enclosed in a thermal core wrap material, a corrugated metallic barrier and two helically wrapped translucent tapes. The foregoing sheath system, which depends on its reflection characteristics to keep heat away from the core, is especially well suited to larger size copper plenum cables.
The prior art has addressed the problem of cable jackets that contribute to flame spread and smoke evolution also through the use of fluropolymers. These together with layers of other materials, have been used to control char development, jacket integrity and air permeability to minimize restrictions on choices of materials for insulation within the core. Commercially available fluorine-containing polymer materials have been accepted as the primary insulative covering for conductors and as a jacketing material for plenum cable without the use of metal conduit. In one prior art small size plenum cable, disclosed in application Ser. No. 626,085 filed Jun. 29, 1984 in the names of C.J. Arroyo, et al. and now U.S. Pat. No. 4,605,818, a sheath system includes a layer of a woven material which is impregnated with a flurocarbon resin and which encloses a core. The woven layer has an air permeability which is sufficiently low to minimize gaseous flow through the woven layer and to delay heat transfer to the core. An outer jacket of an extrudable fluoropolymer material encloses the layer of woven material. In the last-described cable design, a substantial quantity of fluorine, which is a halogen, is used. Fluoropolymer materials are somewhat difficult to process. Also, some of those Fluorine-containing materials have a relatively high dielectric constant which makes them unattractive as insulation for communications conductors.
The problem of acceptable plenum cable design is complicated somewhat by a trend to the extension of the use of optical fiber transmission media for a loop to building distribution systems. Not only must the optical fiber be protected from transmission degradation, but also it has properties which differ significantly from those of copper conductors and hence requires special treatment. Light transmitting optical fibers are mechanically fragile, exhibiting low strain fracture under tensile loading and degraded light transmission when bent with a relatively low radius of curvature. The degradation in transmission which results from bending is known as microbending loss. This loss can occur because of coupling between the jacket and the core. Coupling may result because of shrinkage during cooling of the jacket and because of differential thermal contractions when the thermal properties of the jacket material differ significantly from those of the enclosed optical fibers.
The use of fluoropolymers for optical fiber plenum cable jackets requires special consideration of material properties such as crystallinity, and coupling between the jacket and an optical fiber core which can have detrimental effects on the optical fibers. If the jacket is coupled to the optical fiber core, the shrinkage of fluropolymer plastic material, which is semi-crystalline, following extrusion puts the optical fiber in compression and results in microbending losses in the fiber. Further, its thermal expansion coefficients relative to glass are large, thereby compromising the stability of optical performance over varying thermal operation conditions. Also, the use of fluoropolymers adds excessively to the cost of the cables at today's prices, and requires special care for processing.
Further, a fluoropolymer is a halogenated material. Although there exist cables which include halogen materials and which have passed the UL 910 test requirements, there has been a desire to overcome some problems which still exist with respect to the use of halogenated materials such as fluoropolymers and polyvinyl chloride (PVC). These materials exhibit undesired levels of corrosion. If a fluoropolymer is used, hydrogen fluoride forms under the influence of heat, causing corrosion. For a PVC, hydrogen chloride is formed.
Generally, there are a number of halogenated materials which pass the industry tests. However, if halogenated materials exhibit some less than desired characteristics as required by industry standards in the United States, it is logical to inquire as to why non-halogenated materials have not been used for cable materials. The prior art has treated non-halogenated materials as unacceptable because, as a general rule, they are not as flame retardant or because they are too inflexible if they are flame retardant. Materials for use in communications cables must be such that the resulting cables passes an industry standard test. For example, for plenum cable, such a test is the UL 910 test. The UL 910 test is conducted in apparatus which is known as the Steiner Tunnel. Many non-halogenated plastic materials have not passed this test. Non-halogenated materials have been used in countries outside the United States. One example of a non-halogenated material that has been offered as a material for insulating conductors is a polyphenylene oxide plastic material. Inasmuch as this material has not passed sucessfully industry standard tests in the United States for plenum use, ongoing efforts have heen in motion to provide a non-halogenated material which has a broad range of acceptable properties, as well as a reasonable price and yet one which passes the UL 910 test for plenum cables. Such a cable should be one which appeals to a broad spectrum of customers.
The sought-after cable not only exhibits suitably low flame spread and low smoke producing characteristics provided by currently used cables which include halogenated materials but also one which meets a broad range of desired properties such as acceptable levels of corrosivity and toxicity. Such a cable does not appear to be available in the prior art. Quite succinctly, the challenge is to provide a halogen-free cable which meets the standards in the United States for plenum cables. What is further sought is a cable which is characterized as having relatively low corrosive properties, and acceptable toxic properties as well as low levels of smoke generation and one which is readily processable at reasonable costs.